This invention relates to a short duration timer for use in electrical circuits subject to changes in environmental temperature and supply voltage in which the time duration of the timer is substantially independent of said changes. Such a timer finds particular utility in a glow plug energization control for a diesel internal combustion engine.
Timers are used for a multitude of purposes in electronic circuits. One use for a timer of very short duration is the delay of the actuation of protective shut-down circuitry following a shut-down signal to prevent unintended actuation of the shut-down circuitry by noise or other spurious signals. The duration of the time delay provided is generally substantially less than one second: for example, 0.25 seconds. This time delay must be maintained substantially constant, however, in spite of possible changes in supply voltage and ambient temperature. In a vehicle engine electronic control system, supply voltage, although regulated, does vary and can sometimes fall substantially below the rated voltage, particularly during engine starting in cold weather, when diesel engine glow plug energization control is most useful. In addition, a substantial range of ambient temperatures in the vehicle engine environment are well known to those skilled in the art of vehicle engine control. A short duration timer for use in the protective shut-down portion of the diesel engine glow plug energization control circuit should be well regulated in time duration against changes in supply voltage and ambient temperature.